1. Field of the Invention
The present invention generally relates to optics, and, more particularly, to a passive focus device that modifies (for example, corrects) focus shifts of an optical system during temperature changes.
2. Background Information
Temperature changes in the environment can result in variations in the dimensions of the optical paths in optical devices. Over a given temperature change, different materials will expand or contract at different rates in accordance with the material's coefficient of thermal expansion (CTE). Differing CTE's in an apparatus can result in non-uniform structural changes, thereby impacting optical performance.
Athermalized focus mechanisms can address a focus shift of an optical system during changing temperatures by moving and positioning optical elements to compensate for the shift; such mechanisms can be active or passive in nature. An active motor and drive screw apparatus has been used to provide temperature compensation in, for example, U.S. Pat. No. 5,936,717 (Viola), the disclosure of which is hereby incorporated by reference in its entirety. Additional active athermalizing processes include dynamic focus mechanisms driven by a motor and drive screw with feedback given by temperature sensors and a potentiometer. Actively controlled bellows devices with external pressure controllers are also known.
A passive volume expansion apparatus to provide temperature compensation is described in, for example, U.S. Pat. No. 3,612,664 (Berman), the disclosure of which is hereby incorporated by reference in its entirety. As described therein, linear motion is transferred through a linkage to drive a lens. Additional passive athermalizing techniques include apparatus utilizing a CTE mismatch in solid structures to affect a change in dimensions.
Replacing a dynamic mechanism with a passive device can reduce cost, weight, volume and CCA board space. However, known passive drives are usually limited to small displacements, have limited travel rates, and can exhibit hysteresis problems.